A tree (also known as a Christmas tree) is a complex configuration of actuable valves and other components. They may be used onshore or offshore. Subsea trees are currently operating offshore at every water depth, and are increasingly being used in deeper waters. Additional challenges exist with subsea trees by virtue of being used in a marine environment.
In oil and gas operations, subsea trees may be mounted on top of either injection wells or production wells. An injection well as understood in the art is a well in which fluids are injected rather than produced. Fluid injection into a producing zone of a reservoir is used as an element of reservoir management and may be used to increase oil recovery. The fluids injected into a well may be either liquid or gaseous.
One of the main objectives of injection wells is typically to maintain reservoir pressure or assist in the recovery of oil and or gas by increasing reservoir pressure. Water injection is one type of fluid injection technique that involves drilling injection wells into a reservoir and introducing water into that reservoir, for example, to encourage oil production. Whether water injection occurs before or after production has already been depleted, water injection helps to sweep remaining oil through the reservoir to production wells, where it can then be recovered.
In a production well, produced oil and gas flowing from a reservoir is directed through tubing to the surface and collected for further refining and distribution. A production tree may be useful in controlling and regulating the flow of the oil and gas flowing from a reservoir.
The primary function of a tree is to control the flow of fluids into and out of a well, depending on whether it is an injection well or a production well. However, trees can also include other functionality to allow for troubleshooting, well servicing, etc.